Microphone



April 19, 1927. 1,624,946

H7 0. HAYES MICROPHONE Filed June 7, 1923 Inventor Harvey C. Hu 165 Patented Apr. 19, 1927.

HARVEY C. HAYES, F ANNAPOLIS, MARYLAND.

mcaorno m. I e

Application fled June 7, 1828. Serial No. 644,087.

My invention relates generally to microphones, a-nd more particularly to those adapted to sound receiving equipment, and has for an object the construction of a b microphone which may be employed in a system with a multiplicity of others of the same character and.be evenly matched with such others. 0

Another object is to provide a microphone which is nonresonant.

A further object of the invention 18 to so construct a microphone that packin the carbon granules due to wabbling o the movable electrode is eliminated. With these and other objects in view, the

invention consists in the construction, combination, and arrangement of parts as will be hereinafter described. 1

Reference is to be had to the accompanying drawings forming part of this specification, in which like reference characters indicate corresponding parts throughout the several views, and in which- Figure 1 is a top plan view of my microphone, and

Figure 2 is a longitudinal section thereof. Referring more'particularly to the drawings,'an electrode 5 has secured thereto in any desired manner a'stud 6 on which is carried a. fastening element 7 The fastening element clamps a felt diaphragm 8 between two equal metallic inertia washers 9. The electrode, washers, and the clamping element form the fixed portion of the microphone and these several members are designed have the center of avity of the system he in the center of the elt diaphragm. An electrode 10 carries a stud 11 on which is mounted a nut 12 for clamping the electrode to the body 13 of the microphone. The stud 11 also provides a means for mounting the microphone in place.

The body of the microphone is punched from sheet aluminum, preferably, and is so designed that any resonance that may take place in this member will be of a pitch well above the sound range for which the microphone is designed to operate. The felt d iaphra in, which is highly damped and which has sight restoring force, 15 centered upon and clamped to the body 13 by a light aluminum ring' 14. A felt washer 15 fits loosely around the fixed electrode 5 and is lightly compressed between the body 13 and innermost of the inertia washers 9. as Carbon discs 16 and 17 are in contact with the electrodes 5 and 10 respectively, while the chamber between the discs is partially filled with carbon granules 18, these granules being prevented from working out around the fixed electrode by the felt washer 15. The wall of the chamber is made'an insulator by oxidizing the aluminum surface. The mode of operation isas follows: The microphone being secured by stud 11 to a vibrating member, such as a plate in the side of a ship, when this member vibrates as by sound waves impinging upon it, the whole body of the microphone as well as the electrode 10 takes u the mo- T tlOIl'Of the member to which attac ed, while the electrode 5, being secured to the felt diaphragm 8 and the inertia washers 9, tends to remain fixed, thereby changing the pressure on the body of carbon granules 18, and therefore the resistance to an electrical current flowing through the microphone in accordance with the vibrations of the vibrating member to which the microphone is attached.

Microphones in general use are resonant at one or more frequencies due to the fact that they employ a metallic or mica diaphragm and in some cases due to the fact that the body of the microphone is somewhat flexible. Also when a number of microphones are employed in one system the sensitivity of each is decreased and the sensitivity of the different instruments rendered unequal through the tendency of the carbon granules to pack. This defect has been found to be caused to a large extent by the use of felt or fabric lined granule chambers. It is also caused somewhat when the design is such that the movable electrode wabbles, that is, does not move so that the electrode surface remains parallel with the fixed electrode. Great difiiculty has been ex erienced in matching microphones. This difficulty in a lar e measure is due to the use of resonant iaphragms which are always difficult to match, and the fact that usuall the material of the diaphragm has a di erent temperature coefiicient of expansion than that of the ma- 1 terial of the body of the microphone to which the diaphragm is clamped caused the microphones to become unmatched whenever they are exposed to slightly different temperatures even when they are otherwis matched.

The use of a felt diaphragm, which is highly damped and which has slight restoring force, as well as the design of the body such that its resonant frequencies are above the workable sound range results in giving a practically non-resonant microphone. Uneven matching of the microphones 1S eliminated since the diaphragm is, nonresonant and the felt is sufiiciently flexible to prevent temperature chan es from affecting the characteristics 0 the microphone to any great extent. Wabbfing of the electrode carried by the diaphragm always takes place when the microphone is subjected to vibrations having a direction other than along its axis unless the center of gravity of the inertia suspended by the diaphragm is centered in thediaphragm. The design of my microphone is such that the center of gravity of the inertia does center in the diaphragm, and this construction combined with a carbon granule chamber having smooth aluminum-oxide causes the sensitivity of the microphone to coated sides results in substantially eliminating packing troubles inherent in present types of devices of this nature. The use of aluminum-oxide for insulating the cylindrical surface of, the granule chamber does away with the emission of gases that gradually coat the surface of the granules and decrease with use. Although I have no definite theory as to reasons why this coating of aluminum-oxide has been so successful in preventing packing of carbon anules, and do not wish to limit myse f by any doubtful theory that I may set up, it is thought that the good results are partially due to the smooth surface of the coating, as contrasted with the rough surface of fabric coatings heretofore used, permitting free motion of the moving electrode, tending towards prevention 0 wobbling Having fully described my invention,

what I ire to secure by Letters Patent- 1s:-

1. A microphone having a carbon granule case, the inner walls of said caseb'eing coa ed with aluminum oxide.

2. A microphone having a carbon granule case interiorly coated'with aluminum oxide and a movable element comprising a diaphragm supporting inertia elements and an electrode, the center of gravity of said movable element being centered in said diaphragm.

- 3. In a microphone including a metallic casing, mounting means thereon constituting one electrical terminal for the microphone, a carbon electrode in electrical contact with the mounting means, a nonresonant diaphragm mounted upon the casing, inertia plates upon each side of the diaphragm. clamping means'rigidly clamping the inertia plates to the diaphragm, a second electrode in electrical contact with thephragm, inertia elements on each side there-- of and symmetrically arranged about the center of the diaphragms and'clamping means extending throng the diaphragm and the inertia elements.

6. A microphone having a felt diaphragm, inertia elements one each side thereof, clamping means extending through the diaphragm and the inertia elements, the inertia elements and clampin means so proportioned that the center 0 gravity of the inertia elements and the clamping means lies in the lane of the diafihrggn.

RVEY C. A S. 

